Evidence for a Pathological Ratio of CD44s/CD44v6 in Chronic Lymphoctic Leukemia (CLL) Cells.

Abstract 2628

Poster Board II-604

Chronic Lymphocytic Leukemia (CLL) is the most common adult-onset leukemia in the western world. It is characterized by an accumulation of functionally incompetent monoclonal CD5+ B-lymphocytes and an increased resistance to apoptosis. CD44 is a cell surface transmembrane glycoprotein which has more than ten isoforms generated by alternative splicing. It is expressed on different cells as e.g. cells of lymphohematopoietic origin, epithelial cells and keratinocytes. CD44 is involved in lymphocyte activation, recirculation and homing. It acts as an adhesion molecule and plays an important role in angiogenesis, cell proliferation, differentiation and migration. One of these variants, CD44 variant 6 (CD44v6), is shown to be responsible for an increased apoptotic resistance in Jurkat cells. Our aim was to show that i) CD44 is overexpressed in CLL cells, ii) that this overexpression is regulated by the WNT pathway, which is known to be constitutively active in CLL cells, and iii) that this overexpression is a possible cause for the increased resistance to apoptosis in CLL cells. Peripheral blood of CLL patients was incubated with specific antibodies directed against CD5, CD19, CD44 standard (CD44s) and CD44v6 and measured by flow cytometry. We measured the mean fluorescence intensity of CD44s and CD44v6 on CD5/CD19 double positive CLL cells. The patient pool included patients with poor prognosis ( ZAP70+, CD38+) as well as patients having a good prognosis (ZAP70-, CD38-). CD19 positive B-cells from healthy volunteers, aged from 18 to 60, were analyzed for their expression of CD44s and CD44v6 as well. Expression levels of CD44s in 37 CLL samples and of CD44v6 in 29 CLL samples were measured and compared those of 22 healthy volunteers. We found a high expression of CD44s in CLL-cells (mean: 368.02) but an even higher expression in healthy B-cells (mean: 538.8; p<0.05). In contrast, the CD44 variant 6 was expressed five- to six-fold higher in CLL cells compared to healthy B cells (mean CLL cells: 20.0; mean healthy B cells: 3.6; p<0.001). Taken together, we found that there is a significantly altered ratio of CD44s/CD44v6 expression in CLL cells when compared to normal B cells. As CD44v6 is known to be oncogenic this phenomenon may contribute to the malignant phenotype of these cells. Currently, we are investigating the effect of WNT3a, a WNT signaling initiator, on the expression of CD44v6 in CLL cells and the effect of an anti human CD44v6 antibody on the apoptosis rate in CLL-cells.